1. Field of the Invention
The present invention relates to a noise cancellation system for canceling a noise component for sound reproduced by an audio system from a speech signal input from a microphone, a speech recognition system housing the same, and a car navigation system provided with such a speech recognition system.
2. Description of the Related Art
In the past, a speech recognition system has been known which captures the speech of a user by a microphone, compares it with patterns of speech stored in advance as recognition words, and recognizes high match recognition words as words spoken by the user. This type of speech recognition system is for example built into a car navigation system.
The speech recognition rate of a speech recognition system is known to be governed by the amount of the noise component included in the speech signal input from the microphone. When operating a vehicle-mounted audio system in the passenger compartment of an automobile etc., the music being reproduced by a speaker of the audio system ends up being picked up by the microphone together with the speech of the user as noise. Therefore, in the past, the practice had been to mute the audio system at the time of speech recognition so as to prevent the music etc. from being input to the microphone. (For example, see “Public News Release, ‘Five Car AV & Navigation '99 Summer Models Released’, [online], May 18, 1999, Fujitsu-Ten, [Oct. 16, 2003 search], Internet <URL: HYPERLINK “http://www.fujitsu-ten.co.jp/release/1999/0518.html”, http://www.fujitsu-ten.co.jp/release/1999/0518.html>).
However, in a conventional system of this configuration, since the reproduction of the music etc. ends up being temporarily interrupted at the time of speech recognition, the possibility of the user becoming displeased becomes high. Therefore, in recent years, it has been considered to provide in the speech recognition system a noise cancellation system which acquires the audio signal input to the speaker, estimates the noise component included in the speech signal obtained from the microphone based on this, and cancels the noise component from the speech signal.
As the noise cancellation system, one using a least mean square (LMS) algorithm for the method of estimation of the noise component is known. This type of noise cancellation system acquires the audio signal input to a speaker of the audio system, filters the audio signal by an adaptive filter so as to produce a noise cancellation signal for canceling the noise component from the speech signal, and cancels the noise component included in the speech signal acquired from the microphone based on this noise cancellation signal.
Further, this noise cancellation system is provided with a filter learning unit for changing the characteristic of the adaptive filter (filter coefficient) using the normalized-LMS method (NLMS method, also known as the learning identification method) based on the noise-less speech signal. Due to repeated operation of the filter learning unit, the noise component included in the speech signal is suitably canceled by the noise cancellation signal.
As vehicle-mounted audio systems, audio systems having stereo playback functions (hereinafter also referred to as “stereo audio systems”) have become very widespread, so in vehicle-mounted noise cancellation systems, it is preferable that the noise component produced due to the music etc. being picked up by the microphone can be suitably canceled out. However, with just changing the design of the noise cancellation system to one for a stereo audio system, there was the problem that the entire noise component of the plurality of channels could not be suitably canceled from the speech signal.
As an example of the configuration of a noise cancellation system for a stereo audio system, for example, one providing an adaptive filter and filter learning unit for each channel may be considered (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2001-100784, FIG. 25). In this configuration, filter learning units corresponding to the channels update the filter coefficients based on the noise-less speech signal with residual noise components of other channels, so the learning processes interfere with each other and the filter learning units of the channels mistakenly learn the filter coefficients from the speech signal. Therefore, a noise cancellation system of this configuration cannot cancel the entire noise component of a plurality of channels from a speech signal.
In view of this problem, the speech recognition system of Japanese Unexamined Patent Publication (Kokai) No. 2001-100785 is designed to acquire the audio signals of the channels input to the speakers of an audio system, convert them to a monaural signal, estimate the noise component based on this monaural signal, and cancel the noise component from the speech signal.
Further, Japanese Unexamined Patent Publication (Kokai) No. 10-190848 describes a noise echo canceller method which nonlinearly converts the audio signals of different channels input to speakers of the audio system so as to reduce the correspondence between channels for reproduced sound of an audio system picked up by a microphone and thereby prevent mistaken learning of filter coefficients in the filter learning units corresponding to the channels.
Summarizing the problems to be solved by the invention, the speech recognition system described in Japanese Unexamined Patent Publication (Kokai) No. 2001-100785 estimated the noise component relating to stereo reproduced music etc. based on a monaural signal of the audio signals obtained from the audio system, so had the problem that the estimation error was large and the noise component could not be canceled from the input signal of the microphone with a sufficient precision.
Further, the audio echo canceller method described in Japanese Unexamined Patent Publication (Kokai) No. 10-190848 nonlinearly converted the audio signals of the different channels and used the converted audio signals to produce noise from the speakers, so had the problem that the sound deteriorated in quality. Therefore, if applying this audio echo canceller method to the noise cancellation system for an audio system designed mainly for playing back music etc., the music etc. deteriorated in quality and the user became displeased in many cases.